Coaxial magnetron having attenuator means for suppressing undesired modes



y 3Q 1968 R. E. DECKER 3,395,314

COAXIAL MAGNETRON HAVING ATTENUATOR ANS FOR SUPPRESSING UNDESIRED MODE 2Sheets-Sheet 1 Filed Nov. 24, 1964 INVENTOR Robert E. Decker M 14%ATTORNEY July 30. 1968 R. E. DECKER 3,395,314

COAXIAL MAGNETRON HAVING ATTENUATOR MEANS FOR SUPPRESSING UNDESIRBDMODES Filed Nov- 24, 1964 2 Sheets-Sheet 3 United States Patent COAXIALMAGNETRON HAVING ATTENUATOR MEANS FOR SUPPRESSING UNDESIRED MODES RobertE. Decker, Sunnyvale, Calif., assignor to Westinghouse ElectricCorporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov.24, 1964, Ser. No. 413,586 3 Claims. (Cl. 31539.75)

ABSTRACT OF THE DISCLOSURE The invention relates to a coaxial magnetronhaving an inner resonator system and an outer cavity resonator which isadapted for maximum energy storage of the TE mode. The suppression ofundesired modes in the outer cavity resonator is provided by an energyabsorbing member within a groove provided in the lower surface of theend plate and situated within an annular cavity defined between a grooveon the inner peripheral surface of the end wall and an undercut portionof the anode wall.

This invention relates to electron discharge devices of the magnetrontype and more particularly to coaxial magnetron type.

In the R. J. Collier and J. Feinstein U.S. Patent 2,854,603 issued Sept.30, 1958, there is described a coaxial magnetron structure whichcomprises an inner and outer resonant system. The inner resonant systemincludes a cylindrical anode wall together with a plurality of anodevanes radially extending inwardly therefrom. These vanes define acircumferential array of inner, or anode cavity resonators. An outercavity resonator is defined between an outer wall and the cylindricalanode wall and forms the outer resonant system. The two systems, namelythe inner resonant system and the outer cavity resonator, are coupled bya circumferential array of slots provided in the cylindrical anode wallwhich connects the outer resonant system with the inner resonatorsystem. The inner resonant system is normally designed to oscillate inthe pi-mode, while the outer system is dimensioned for maximum energystorage of the TE mode. The two systems are effectively locked togetherby the coupling slots provided between the inner resonant system and theouter cavity resonator.

The TE mode in cylindrical and coaxial cavities is often called thecircular electric mode since the electric field exists only in acircular pattern and there is no radial or axial directed electricfield. There are other modes that can exist in the right circularcoaxial cavity as well as the TE such as TE TE and TE In addition TMmodes also can exist within the cavity. These modes may be separatedinto competing and interfering modes. The TE modes are the mostimportant since they couple into the vane structure of the innerresonant system. The TE mode coaxial cavity generates a pi-m ode. In alike manner the TE mode generates a pi-l mode field. As previouslyindicated, the current in the outer cavity in the TE mode is everywherecircumferentially and tends to be stronger at the central regions of thecylindrical an ode, the outer cylinder and the end covers therefor. Thecurrents at the corners of the cavity are very small, andcircumferential. In the TE mode, which is the most potential competingmode, currents move axially and radially and there are strong currentsmoving across the junctions of the cylindrical anode cylinder and theend covers. If one of these junctions is broken, it is possible to matcha damping resistor or attenuator to the undesired TE mode with virtuallyno damping on the desired TE mode. The net effect is to greatly lowerthe TE 3,395,314 Patented July 30, 1968 mode Q to the point where itwill not sustain oscillations.

Thus, in the previously mentioned Collier and Feinstein patent a quarterwave circular choke was provided Within an end cover adjacent to andcoaxial with the cylindrical anode wall. The interior of the choke waslined with soft iron to provide magnetic losses.

Another problem associated with the coaxial magnetron is providingproper cooling of the tube. The vane tips which are subject to electronbombardment are the members primarily heated. The heat must flow throughthe vanes, the anode cylinder, the end covers and finally to the heatradiator which is positioned on the outer wall of the cavity resonator.It is therefore important that a continuous thermal path of good thermalconductive material be provided from the vane tips to the heat radiator.This heat problem must be considered in determining the propersuppression means used within the magnetron. The heat flow path normallyis provided particularly in a tunable type cavity through the bottom ofthe anode structure. The top of the anode should be undisturbed toprovide a top locating surface.

It is accordingly an object of this invention to provide an improvedelectron discharge device.

It is another object to provide an improved coaxial magnetron structure.

It is another object to provide an improved mode attenuator forcompeting modes in a coaxial magnetron.

It is another object to provide an improved mode suppressor for the TEmode within the cavity of a coaxial magnetron.

Briefly, the present invention provides a coaxial magnetron having aninner resonator system and an outer cavity resonator which is adaptedfor maximum energy storage of the TE mode. This suppresssion isaccomplished by provision of an energy absorbing member within a groovethat is below the surface of the end cover and extends below the surfaceof the cylindrical anode.

Further objects and advantages of the invention will become apparent asthe following description proceeds, and features of novelty whichcharacterize the invention will be pointed out in particularity in theclaims annexed to and forming a part of the specification.

For a better understanding of the invention, reference may be had to theaccompanying drawings, in which:

FIGURE 1 is a perspective view, partially in section and partiallybroken away of a coaxial magnetron illustrating one specific embodimentof the invention; and

FIG. 2 is a side view in elevation of the device in FIG. 1 to whichappropriate magnets have been added.

Referring now to the drawings, there is shown a tunable coaxialmagnetron. The coaxial magnetron is comprised of a body member 10 whichis substantially cup-shaped and includes a cylindrical side wall orouter wall 12 and a bottom end plate or cover 14. The mamber 10 is of asuitable electrically and thermally conductive material such as copper.Heat radiating fins 13 are provided on the outer surface of the outerwall 12.

Centrally disposed with the body 10 is a cathode 16 which includes asleeve member 18 of a suitable material such as molybdenum. The sleevemember 18 is provided with an electron emissive coating 20 on the outersurface. Suitable heating means (not shown) is provided with the cathodesleeve 18.

A cylindrical anode 22 of a suitable electrically and thermallyconductive material such as copper surrounds the cathode 16. A pluralityof radially extending vanes 24, also of suitable conductive material,are provided on the inner surface of the anode cylinder 22. The vanes 24extend axially along a small portion of the length of the anode cylinder22. The vanes 24 define an array of inner cavity resonators 26. Slots 28extends along the anode cylinder 22 for a greater length than the vanes24 and 3 are substantially parallel to the vanes 24. The slots 28 arelocated in alternate anode resonators 26 and provide communicationbetween the inner cavity resonators 26 to an external or outer cavityresonator 30.

The cylindrical wall portion 12 of the body 10, the cylindrical anode22, the bottom end plate 14 and an annular tuning member 44 define thecavity resonator 30. Extending through the wall 12 to communicate withthe outer cavity resonator 30 is an output coupling opening 32. Theopening 32 serves as means through which energy may be removed from theouter cavity resonator 30 and has been shown in its simplest form.

Positioned atop the cup-shaped body member is a substantially diskshaped cover means 34. The cover 34 is of a suitable non-magneticmaterial such as stainless steel and is vacuum sealed at its peripheryto the cylindrical wall portion 12. The inner periphery of disk shapedend cover 34 is sealed to a pole piece 50.

Tuning is provided in the coaxial magnetron shown by axially moving theannular member 44 within the outer cavity resonator 30. In the specificembodiment shown, this conductive member 44 is an annular member of asuitable electrically conductive material such as copper. The member 44defines one boundary surface of the cavity resonator 30. The tuningmember 44 is substantially U-shaped in cross sectional area. The tuningmember 44 is actuated by means of two rod members 64 which extendthrough suitable apertures 66 within the end cover 34. The rods 64 haveone end fixed to the annular member 44 and the other end fixed to across bar member 62. The upper pole piece 50 is provided with anextended portion 68 which extends through an aperture 70 centrallylocated within the cross bar member 62 and into a centrally extendingbore 72 within an actuating rod 60. The rod 60 is attached to the crossbar member 62 and is slidably mounted in a sleeve member 58 surroundingthe rod 60. The sleeve member 58 is secured to a magnetic spacer member52. The rod 60 may move within the sleeve 58 and provide movement of theannular member 44 to adjust dimensions of the cavity resonator 30. Inthis manner, tuning of the magnetron is accomplished.

The magnetic circuit in the present device shown in FIG. 2 is inclusiveof two substantially identical horseshoe magnets 46, a bottom pole piece48 and the upper pole piece 50. Also included in the magnetic circuit isthe magnetic spacer 52 which is substantially a U-shaped member in whichserves to connect the upper poles of the magnets 46 to the upper polepiece 50. The magnetic spacer 52 also serves as a support means for thetuning drive mechanism. Pole pieces 48 and 50 and spacer 52 may be of asuitable magnetic material such as soft iron.

The outer cavity resonator 30 is capable of sustaining a number ofdifferent modes of operation. In this particular application, the cavityresonator 30 is dismensioned to provide maximum storage of the TE modeat the operating frequency. The electric currents flow circumferentiallyon the outer surface of the anode wall 22 and along the inner surface ofthe outer wall 12 of cavity resonator 30. The inner resonant systemdefined by the resonators 26 will tend to oscillate in both the pi andvarious degenerate modes. Thus, the outer cavity resonator 30 and theinner anode resonators 26 can be considered as two distinct resonantsystems; however, when the two systems are placed together they can beconsidered to be a single composite system. Current produced by theouter cavity resonator mode flows along the anode wall 22 in a directionperpendicular to the slots 28. The anode vanes 24 are approximately aquarter wave length in radial length at the operating frequency suchthat the high impedance termination at the free end of the anode vanes26 is reflected back to the slots 28 as a low impedance and accordinglythe electric current flows into the resonator 26 and down the adjacentvane 24. These high voltages appear across alternate anode vanes 24 at agiven time because only alternate inner cavity resonators 26 are coupledby the slots 28 to the outer cavity resonator 30. Voltages at otheralternate anode vanes are provided by mutual inductance resulting insuch voltage being out of phase with the adjacent vanes. This is theproper condition for maintenance of the pi-mode oscillation with theinner resonant system. The electron beam of the inner resonant systemdue to the crossed electric and magnetic fields induces voltages in thetips of the vanes 24. These voltages in turn produce currents which fiowout through the slots 28 into the outer cavity resonator 30. Theresonators 26 are normally constructed so as to support a frequencyoutside the frequency range of the outer cavity resonator 30 such thatthe natural resonant frequency of the inner structure has little effecton the tuning of the outer cavity or of the frequency of oscillation ofthe outer cavity.

The outer cavity resonator 30 is capable of sustaining a number ofdifferent modes of operation. In accordance with this invention,however, it is dimensioned as well known in the art, for maximum storageof the TE mode. In this type of mode, the magnetic field lines areradially along the upper and lower plates of the cavity resonator 30 andthe electric field lines are entirely circumferential. Electric currentsinduced within the external cavity resonator 30 fiow circumferentiallyalong the outer surface of the anode wall 22 along the inner surface ofthe outer wall 12 the inner surface of the tuning element 44 and the endplate 14. Currents induced by other modes such as the TE will have anaxial component which may be damped by this invention. This is shown inFIG. 1. The anode cylinder 22 is reduced in its outer diameter at thelower end portion to provide an annular groove 43 at a point just belowthe slots 28. The thickness of the anode wall 22 at the groove 43 isabout one-half the thickness of the remaining portion of the anode wall22 which has a thickness of about .060 inch. The lower annular end plate14 is to provide an aperture 45 for receiving the lower pole piece 48.The surface of the aperture 45 is provided with an annular groove 47.The groove 47 has a lower lip 49 and an upper lip 51. The diameter ofthe lip 49 is substantially the same as the inner diameter of the anode22. The lower edge of the anode 22 is brazed to the upper surface of thelip 49. The upper lip 51 has a diameter greater than the outsidediameter of the anode 22, and defines an annular gap 55.

The upper surface of the lip 51 is in the same plane as the remaininginner surface of the end cover 14 and is above the groove 43 in theanode wall 22. The annular groove 43 opposed the annular groove 47 anddefines a region or cavity within the junction of the anode 22 and theend cover 14 wherein an annular ring 57 suitable lossy material such ascarbon impregnated alumina, silicon carbide, or resistive film coatedceramic may be positioned. It is found that by making the thickness ofthe lip oprtion 51 of a thickness of about .010 inch and the annular gap55 between the lip 51 and the outer diameter of the larger portion ofthe anode 22 of about .010 inch the resulting TE mode unloaded Q is verylow with little effect on the unloaded pi-mode Q. The heat flow pathfrom the vanes 24, through anode cylinder 22 is still provided by meansof the restricted portion of the anode 22 at the groove 43 and throughthe end plate 14 to the outer cylindrical wall 12.

The radial currents of the TE anode cannot jump the gap 55 and areattenuated as 1 R losses as they pass around the walls of the grooves 43and 47 holding the attenuator ring 57. The cutback in the diameter ofthe anode 22 and the long thin lip 51 increases the discontinuity of thecurrent path. This utilizes nearly the entire surface of the attenuatorring 57 which fits into the copper cavity formed by grooves 43 and 47.Even though there is little pi-mode current in area, narrowing the gap55 increases the pi-mode unloaded Q by making the cavity appear moreperfect at the anode cavity junction.

The long narrow lip 51 or the cutback anode alone will aid inattenuating the TE mode but the combination thereof achieves the mostdesirable result.

While there have been shown and described what are presently consideredto be the preferred embodiment of the invention, modifications theretowill readily occur to those skilled in the art. It is not desired,therefore, that the invention be limited to the specific arrangementshown and described and is intended to cover in the appended claims allsuch modifications as fall within the true spirit and scope of theinvention.

What is claimed is:

1. A coaxial magnetron comprising a cylindrical anode wall member, aplurality of vanes extending inwardly from said anode wall member anddefining a plurality of anode cavity resonators within said anode wall,a cathode positioned centrally of said anode wall member, the outersurface of said anode wall member defining one Wall of an annular outputcavity resonator, said output cavity resonator coupled through openingsin said anode wall to said anode cavity resonators, said outer cavityresonator including a lower annular end wall member having a portion ofits inner periphery secured to the lower end of said anode wall member,said anode wall member having a reduced outer diameter portion about onehalf the thickness of the remaining portion of said anode wall memberand below the upper surface of said end wall member, the upper surfaceof said end wall member provided with a first annular opening therein toan annular chamber, said annular chamber defined by said reduceddiameter portion and a groove provided in the inner periphery surface ofsaid annular end wall, the wall between said chamber and said outputcavity resonator within said end wall having a thickness of about .010inch and an attenuator means positioned within said chamber.

2. A magnetron comprising a cylindrical wall member, a plurality ofvanes extending inwardly from said wall member and defining inner cavityresonators, a cathode positioned centrally of said wall member, meansincluding said Wall member defining an outer cavity resonatorencompassing said inner cavity resonators, means for coupling said innercavity resonators to said outer cavity resonator, said outer cavityresonator including a lower end plate member secured to the lower end ofsaid Wall member, said wall member having a reduced outer diameterportion to provide a wall thickness of about one half the thickness ofthe remaining portion of said wall member, at the lower end thereof,said reduced outer portion below the plane of the inner surface of saidlowerend plate and an attenuator ring positioned about the reduceddiameter portion of said wall member and below the plane of the innersurface of said lower end plate member.

3. A magnetron comprising a cylindrical. anode member, a plurality ofvanes extending inwardly from said anode member and defining anodecavity resonators, a cathode positioned centrally of said anode member,means including said anode defining an outer cavity resonatorencompassing said anode cavity resonators, means for coupling said anodecavity resonators to said outer cavity resonator, said outer cavityresonator including a lower end plate member electrically and thermallyconnected to the lower end of said anode member and forming a junctiontherewith, said anode having a reduced outer diameter surface below theupper surface of said lower end plate member, an annular slot providedin the upper surface said end plate member, said annular slot having adiameter greater than said outer surface of said anode wall member, saidannular slot communicating with an annular cavity within the junction ofsaid anode wall and said end plate member beneath the upper surface ofsaid end plate member, the walls of said chamber defined by the reduceddiameter surface portion of said anode wall and a groove in the surfaceof said end plate member facing said reduced outer diameter portion ofsaid anode member and an attenuator ring positioned within said annularcavity.

References Cited UNITED STATES PATENTS PAUL L. GENSLER, PrimaryExaminer.

